a cmos 8x8 spad array for time-of- flight measurement and ... · a cmos 8x8 spad array for...

WASC, June 3-4, 2013, Seville 1

I. Vornicu, R. Carmona-Galán, Á. Rodríguez-Vázquez

A CMOS 8x8 SPAD Array for Time-of-Flight Measurement and Light-Spot

Statistics

Institute of Microelectronics of Seville (IMSE-CNM)CSIC-University of Seville

Workshop on Architecture of Smart Camera (WASC), Seville, Spain

June 3-4, 2013


Outline

• SPADs in Positron Emission Tomography

• 8 x 8 SPAD array for ToF and spot characterization

• Architecture

• Functionality

• Simulation results

• Conclusions


Creates images by measuring electro-magnetic radiation emitted by the radiotracer molecule

Positron emission tomography (PET)

Single photon emission tomography (SPECT)

[Cherry 2003][Wernick 2004]

The key element to the successful nuclear medicine imaging is proper selection of the radioactive tracer [Saha 2005].

Nuclear medicine

PET: principles of operation

PET combines diagnosis with treatment

[Iniewski 2009]

An atomic electron Positron annihilation = interacts with

a positron


The detection of the two events establishes a line-of-response (LOR)

PET: principles of operation

The required time (Tbin) and spatial resolution are below few nanoseconds and

4x4mm² respectively

LOR

rayγ

Detection improvement byMinimizing the uncertainty along LOR by ToF estimation

Increasing spatial resolution by detecting the position of the maximum spot

Increase the diameter of detectors rings

Improve spatial resolution (LSS)

[Iniewski 2009]

Minimize parallax error


Depth of interaction (DOI)γ-ray

Visible photons

High-energy photon

DOI

Scintillatorcrystal

• Inaccurate determination of DOI results in incorrectly positioned LOR

• Incorrectly positioned LOR leads to parallax errors and, hence:

• Imprecise reconstruction of the image

• DOI in continuous scintillatorcrystals can be inferred from width of light-distribution

[Kao et al. 2000]

[Hoffman et al. 1989]

[Lerche et al. 2005]


Aggregated row and column detections

Centre of mass of the light spot:

Variance (second moment):

Relation between DOI and variance:

Focal-plane calculation of the DOI

∑=

=N

iiip

11μ

( )∑=

−=N

iipi

1

21

2 μσ

( )200 σσ −∝− zz[Pozas et al. 2011]


D1=L+d; T1=D1/c;

∆T=2d/c – is the time difference in detection between detector 1 and 2.

D2=L-d; T2=D2/c;

[Lewellen 1998]

[Conti 2008] Less data is needed to infer the actual position of the radio tracer

ToF minimize uncertainty along LOR

Tbin=200ps, ∆x=3cm


PET detectors: PMT vs. SPAD

Photomultiplier tubes:mature technology, widely employedaffected by magnetic fields

limited spatial resolutionmultiple crystal arrays are expensiveundetermined DOI

Single-photon avalanche diodes:experimentalcan operate in magnetic fields

better spatial resolutioncheaperfocal-plane image processing

rayγ

visible spectrum


DV

DI

1

2

3

Single-photon avalanche diode

1) A photodiode biased beyond breakdown stays at zero current until avalanche is initiated

2) Avalanche can be triggered by one single photon absorption event, DC or AP

3) Quenching circuit (active or passive) limits the avalanche current by keeping |V0-VA|<VBD

Avalanche must be quenched to avoid device destruction

The simplest scheme is passive quenching

Active quenching and recharge can be employed to decrease dead time

DV+

−DI

2

3 11

Passive quenching

Active quenching

1) Dark counts

2) After-pulsingUnwanted events


State of the art

Ref.

#

Techno

logy

DCR Time resolution

Pixel size Array size Incorporated functionality

Tunable

dead time (DT)

VbiasSPAD

[Tisa et al. 2008]

CMOS 0.35um

Up to 30Khz

617ps 50 x 100um - Tofmeasurements

40ns –2us

24V

[Fara. et al. 2008]

CMOS 0.18um

40Khz - 90 x 100um - Tofmeasurements

30ns 10.2V

[Braga et al. 2011]

CMOS 0.35um

1-2Khz expected

325ps 145 x 215um

14x10 mini SiPM with 32 SPADs

Data compression for

PET

- -

This work

CMOS 0.18um

25kHz 150ps expected

32 x 32um 8 x 8

Tofmeasurements

Statistics of the spot for

PET

3ns –40ns

10.8V


Features double functionality:ToF measurements Focal plane statistics

Array architecture

Row

cou

nter

Col decoderR

ow d

ecod

er

Col counters

Data serialiserD

ata

seria

liser

8x8 SPADs +AQR

Row

cou

nter

Col decoderR

ow d

ecod

er

Col counters

Data serialiserD

ata

seria

liser

8x8 SPADs +AQR

CMOS SPAD


Array architecture

Vspad+K

A

AQR

GNDspad+

Vspad+K

A

AQR

GNDspad+

Vspad+K

A

AQR

GNDspad+

Vspad+K

A

AQR

GNDspad+

ENRST

ENRST

ENRST

VDD VDD

OVFB12

B1

OVFB12

B1

A. Time of flight configuration

Each SPAD is multiplexed to a single output

B. Light spot statistics configuration

MAveraging M measurements, TR improves by

Time gated measurement

Row/column counters are collecting data to build up statistics of the actual position

of the impinging light beam

Very low light illumination


Active quenching and recharge

AQR – this work

AQR principle [Tisa 2008]Variable-load quenching circuit [Tisa 2008]

AQR principle [Faramarzpour 2008]


Post layout simulations

0

0.5

1

1.5

2 /Vout pixel /Vout array /Vout PAD

Vol

tage

(V)

348 350 352 354 356 3580

0.5

1

1.5

/Anode current spike

Cur

rent

(mA

)

time(ns)

pixelarrayPADT1=374ps

T2=2.8ns

Worst case signal path delay

The output buffer introduces an systematic error

The delay (T1) introduced by the AQRcircuit is much smaller than T2

a) Shape of the output voltage along the signal pathb) Current spike that flows through the SPAD

SPAD are diodes working beyond the breakdown (BD) region

Fast quenching circuits are mandatory to avoid device destruction



0

0.5

1

1.5

2/Vreset /VA

Vol

tage

(V)

248 249 250 251 252 253 254 255 256 2570

0.5

1

1.5

/I(A)-anode current spike

Cur

rent

(mA

)

time(ns)

Vreset

VA

Active quenching

Active reset

Due to the current that flowsthrough M3 and M4

VA is the anode voltage

Vreset is a very short pulse (~300ps)it restores the state of the SPADthat is ready to trigger another avalanche



0

0.5

1

/Vcap

0

1

2/Vout

Vol

tage

(V)

250 255 260 265 270 275 280 285 2900

0.5

1

1.5/I(A)-anode current spike

Cur

rent

(mA

)

time(ns)

DT=5.6ns DT=41ns

DT=5.6nsDT=41ns

Unwanted pulses can be triggered by:- dark count events (DC)- after pulsing events (AP)

AP rate can be decreased by increasingDT

DT of the SPAD can be tuned between 4 and 41ns

DCR and AP are also depending on theexcess voltage (Ve)

Lower sensitivity

Ve


8x8 SPAD array (ToF and Spot Stats)1.5mm

1.5m

m

CMOS technology UMC 0.18um

No. of SPADs 8 x 8 test structure

Die area 1.5mm x 1.5mm

Sensor area 256um x 256um

SPAD cell pitch 32um x 32um

SPAD diameter 12um (14um external TWELL)

Counters depth 13b

Time resolution <150ps


8x8 SPAD array (ToF and Spot Stats)32um

32u

m

CMOS technology UMC 0.18um

No. of SPADs 8 x 8 test structure

Die area 1.5mm x 1.5mm

Sensor area 256um x 256um

SPAD cell pitch 32um x 32um

SPAD diameter 12um (14um external TWELL)

Counters depth 13b

Time resolution <150ps


Preliminary results

The output of one Individual cell

Laser modulating signal

Events triggered by the laser

Events triggered by DC, AP and minimum illumination power

The light beam reaches themaximum power level

The light beam reaches theminimum power level


Conclusions

• We have designed an array of SPADs in order to improve accuracy in PET imaging

• Rows and columns of the array behave as independent digital Silicon Photo-Multipliers.

• The outcome of the SiPM processing are the histograms of the rows and columns of the array.

• This information is useful to compute the DOI and therefore to achieve a clearer picture of the sample.

a cmos 8x8 spad array for time-of- flight measurement and ... · a cmos 8x8 spad array for...

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